In recent years, facsimiles, printers, and the like have been developed markedly, and in particular, a heat-sensitive recording method comprising using a thermal head to record on a heat-sensitive recording sheet comprising a colorless dye such as crystal violet lactone or the like and a phenol compound as disclosed in U.S. Pat. No. 3,539,375 and the like has been widely adopted.
The heat-sensitive recording method has many advantages in that the recording sheet is for primary coupling (requiring no development), the recording apparatus can be simplified, the recording sheet and recording apparatus are inexpensive, and the recording method is of the non-impact type and is not noisy, so that it has established its position as a low speed recording method. However, a large defect of the heat-sensitive recording method is that its recording speed is lower than that of the other recording methods such as electrostatic recording and the like, so that, prior to the present invention, the heat-sensitive recording method was not adopted for high-speed recording.
The greatest reason why high-speed recording could not be conducted using the heat-sensitive recording method was that heat conduction was not sufficiently carried out between a thermal head and the heat-sensitive recording sheet brought into contact with the thermal head and a sufficient recording density could not be obtained.
A thermal head having dot-like electric resistance heating elements generates heat according to recording signals which thereby causes melting and coloring of the heat-sensitive coupling layer which is contact in with the thermal. To obtain clear, deep-density recording, it is necessary that dot reproducibility is good, that is, the thermal head is contacted with the heat-sensitive coupling layer as fast as possible to carry out heat conduction efficiently, and dots colored completely and corresponding to the shape of dot heating elements in the thermal head are formed, completely corresponding to the high-speed recording signals, on the heat-sensitive coupling layer. Prior to the present invention, however, only several percent of heat quantity generated in the thermal head is conducted to the heat-sensitive coupling layer and the efficiency of heat conduction is very low.
Some methods of improving the evenness of the heat-sensitive coupling layer to contact a thermal head to the heat-sensitive coupling layer as fast as possible have been proposed.
In Japanese Patent Publication No. 20142/77, the surface treatment of heat-sensitive coupling layer having a Beck smoothness of 200-1000 sec is disclosed, and in Japanese Patent Application (OPI) No. 115255/79 (the term "OPI" as used herein means an "unexamined published application), it is disclosed that a heat-sensitive coupling layer of Beck smoothness of 200-1000 sec can correspond only to a heat pulse of about 5-6 milisecond and that the surface of a heat-sensitive coupling layer needs to be surface-treated to a Beck smoothness of 1100 sec or more to carry out high-speed recording with a heat pulse of 1 milisecond or less. When the Beck smoothness is 1100 sec or more in the heat-sensitive coupling layer, a colored fog is formed by pressure, so that the smoothness of base paper to be used is maintained at a Beck smoothness of 500 to 800 sec in advance to prevent formation of colored fog. In Japanese Patent Application (OPI) No. 156086/76, it is mentioned that the surface roughness, Ra, of a heat-sensitive coupling layer should be 1.2 microns or less and the glossiness of the surface of the layer 25% or less. (The glossiness means specular glossiness measured at the angle of incidence of 75.degree. as describedin JIS-P-8149.)
In the conventional techniques for improvement in the above-mentioned evenness, the smoothness of heat-sensitive coupling layer has been improved only by calendering treatment with a super calender, a machine calender, a gloss calender, or the like. The calendering treatment is applied only to the base paper, to the base paper and heat-sensitive recording sheet, or the heat-sensitive recording sheet.
Sticking is a phenomenon of the thermal head sticking to the heat-sensitive coupling layer and as a resulting of sticking, a release noise is generated or dot reproducibility is reduced. Piling is a phenomenon of the heat fused material of heat-sensitive coupling layer piling up in the thermal head and as a result of piling, a recorded density and dot reproducibility are both reduced. Any of the two phenomena hinders stable recording.
As a heat-sensitive recording sheet is improved in its smoothness and in its recorded density, the sticking and piling phenomena of the heat-sensitive recording sheet are increased, so that actually, the smoothness is restrained to an appropriate level so that the recorded density is ballanced with the sticking and piling phenomena. Even if the smoothness of heat-sensitive recording sheet of the conventional techniques is set at any possible level, the heat-sensitive recording sheet is not practical for high-speed recording in respect of recorded density or recording stability.
Further, another defect of heat-sensitive recording sheet caused by the calendering treatment is that a colored fog is formed by pressure and the density of textured portion of the recording sheet is increased. In contrast, when base paper is calendering-treated, so-called depressed areas, or bonds wrinkles caused by uneven weighing are formed, and the treatment of such defect is limitative.
As mentioned above, improvement in smoothness of heaat-sensitive coupling layer and in the recorded density of the recording sheet by the calendering treatment is necessarily limited and satisfactory heat-sensitive recording sheet for high-speed recording has not been obtained by the calendering treatment.